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Ear

Ear

The ear detects external vibrations, changes in the motion and position of the head, and acceleration or deceleration of the body. The ear is divided into three areas: external ear, middle ear, and inner ear.

The visible part of the ear, that is, the auricle and the external auditory meatus, are considered the external ear. The auricle is shaped to help funnel vibrations into the ear and is composed of hyaline cartilage. The external auditory meatus is lined with modified sweat glands which secretes cerumen, a wax like substance. The meatus is not a straight tube, but it can be straightened by pulling the auricle up and back.

The external ear is separated from the middle ear by the tympanic membrane, or "eardrum."

The middle ear houses the smallest bones found in the human body: the malleus, incus, and stapes (hammer, anvil, and stirrup). The malleus is attached, on one side, to the tympanic membrane; the other side is connected to the incus. The incus, in turn, connects to the stapes, which is connected to the fenestra ovalis, or "oval window."

The inner ear is separated from the middle ear by the oval and round windows. It is composed of a membranous labyrinth, the semicircular canals, utricle, saccule, and cochlea, and is surrounded by a bony labyrinth which provides protection. The inner ear has three semicircular canals, each in a plane at approximately right angles to the others. Near the junction with the utricle, the canal enlarges into an ampulla.

The ampulla houses receptor cells, which detect head movement. The utricle and saccule are membranous sacs with areas called maculae which have fine receptor hair cells attached to a granule of calcium carbonate. The entire macula responds to gravitational pull, linear acceleration, and position changes. These two organs, the utricle and saccule, are involved in sensing equilibrium.

The cochlea is composed of three tubes: scala vestibuli, scala tympani, and cochlear duct, which are coiled into a snail-shaped duct. The scala vestibuli, which ends at the oval window, and the scala tympani, which ends at the round window, are joined together at the top of the cochlea. Both are filled with endolymph. The cochlear duct contains the organ of Corti which consists of highly specialized vibration receptor cells.

Sound travels down the external auditory meatus and oscillates the tympanic membrane, which causes the bones to vibrate. The bones in turn vibrate the oval window, stimulating the receptors in the organ of Corti which produces neural transmissions to the brain. The brain interprets these signals as sounds.


References
Antony, C.P. & G.A. Thibodeaw. 1979. Textbook of Anatomy and Physiology. The C.V. Mosby Company, St. Louis. 731 pp.

Gray, H. 1977. Gray's Anatomy. Crown Publishers, Inc, New York. 1257 pp.

Lockhart, R.D., G.F. Hamilton, et. al. 1974. Anatomy of The Human Body. Faber and Faber Limited. London. 697 pp.

Van Amerongen, C. The Way Things Work; Book Of The Body. New York: Simon and Schuster, 1979.